Film-Tech Forum ARCHIVE: Spacing Between Amplifiers

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Posted by James R. Hammonds, Jr (Member # 673) on 05-04-2001, 04:57 PM:

Does anyone have any suggestion on how much space should be left between each amplifier in the sound rack?

My theatre uses CROWN COM-TECH amplifiers and we have had many problems with channels going out.

My manager thinks that they might be getting too hot because they are stacked on top of each other, but I didn't think it mattered since there is no ventilation on the top or bottom; just on the front.

Our booth is at a proper temperature, so I don't think that is the problem.

Any ideas?

Posted by Ken Layton (Member # 133) on 05-04-2001, 05:11 PM:

I like to leave one rack space between amplifiers, if there's room in the rack to do so.

Posted by Ian Price (Member # 14) on 05-04-2001, 05:11 PM:

If they don't have any ventalation holes in the top or bottom then they are designed to be placed in the rack with no spacing. If you have to room in the rack, you may move them one rack space apart. It can't hurt.

As you can see from our photos, I am only one rack space high, where as Ken Layton is at least four rack spaces high.

At another theatre we used to use an air compressor and blow air in the amps and hideous amounts of dust would fly out the other side. I'm sure that dust could be an amp killer.

Posted by Randy Stankey (Member # 64) on 05-04-2001, 05:22 PM:

I think poor ventillation is a possibility. It can't hurt to give the amps more breathing room.

However, I'd check your speaker hookups for the proper impedance.
I know of several theatres that have their speakers (especially the surrounds) hooked up either all in series or all in parallel. This can cause a borderline overload (or sometimes an "underload") on the outputs which will cause the amps to overheat and fail prematurely.

What you should do is check the specs of the amplifiers and make sure the impedance of the speaker hookups matches the amps' specs as closely as you can manage. This means that you'll have to hook the speakers up in some kind of series-parallel arrangement. Yes, you'll have to get out your calculator and a pencil and paper and do y'erself some good ol' fashioned cipherin'. If you have an odd number of speakers you might only be able to get the impedance to come out to 6.667 ohms but that's a whole lot better than some wierd value like 3.334 ohms.

Even though some amps say they can tolerate loads as low as 2 ohms I don't think they mean that they can do it all the time. Besides they'll run a whole lot cooler and they'll probably sound a little bit better as well.

PS: Why can't this board parse the ohms symbol? (?) -- Just wondering.

Posted by Darryl Spicer (Member # 711) on 05-04-2001, 05:57 PM:

I believe you shoud have space between each amp that is eaqual to half the hieght of the amp. Even though there is no vents on the top does not mean u can stack them on top of each other. Amps get extreemly hot inside and the fans just do not pull out all the heat. An amp problem can occure do to the heat disapating of the metal cases. heat rises so it rises right up into the next amp causing the internal heat to go up even more. this causes the amp to go into thermal protection mode.

Hey Randy,
Heard you have been spending some time at the Saulisbury Tinseltown. Don not giving you a hard time is he. He not saying any lies about me is he. LOL well, next time you see him tell him I said hello and I'll email him soon.

Posted by Scott Norwood (Member # 30) on 05-04-2001, 07:02 PM:

What does the Crown manual say? It should have a section on cooling and installation that tells you how close they can be stacked.

Most of the QSC manuals, for example, claim that their amps can be stacked without space between them. If that is the case, then you "should" be able to trust that, although there would certainly be no downside to allowing for more space between the amps if you have the rack space, especially if they run hot.

We had Crown amps at my college (the same place that had the 16mm Eikis) and they were racked without extra space and didn't seem to have a problem, although they didn't get the kind of use that they would in a commercial movie theatre.

Posted by Pete Lawrence (Member # 130) on 05-04-2001, 07:41 PM:

I have three Crown Com-Tech amps in my home screening room booth. The top and bottom panels are just sheet metal covers. There are no active parts mounted on the covers. It should be no problem stacking them together. A real problem might be in that the cooling air enters through a grill in the front and is exhausted along both sides of the amp INSIDE the rack. If the entire rack is not vented well everything is going to get hot. You might consider a fan near the top of the rack to pull out the excess heat. I don't run mine hard enough for the fans to come on most of the time, they only run when the heatsinks get warm enough.

Posted by Steve Guttag (Member # 268) on 05-04-2001, 09:47 PM:

The Crown Com-Techs can be stacked right on top of one another. It has thermal sensors on the heat sinks to control the fan. The fan draws in from the front (hopefully through a routinely cleaned filter) and blows out the sides.

I have to repair quite a few of the Crowns based on that platform (Macro, Micro and Com-Tech)....their failure mode can be rather nasty...if the output devices fail...the will often take out the main module as well. Crown also tends to beta test in the field...if you have an early version of their amplifier then they are still in the testing phase...If you ever look at their service manuals....there are often several revisions until the settle on the final version.

Crown also often will settle for a marginal componet in their amplifiers...that is, if the theoretical wattage for a resistor is .497 watts....they will just use a 1/2 watt resistor...I'm sure some bean counter can show the cost savings and the amplifier will make it out of warranty before it fails.

As to impedance...the Com-Tech should deal with 2-Ohms just fine btw.

Randy, you are incorrect in that many amplifiers nowadays will run just fine into 2-Ohms...in fact, the QSC DCA-1222 will run just fine into 1.6 Ohms...all day long! Looking at the spec sheets you can often tell if it really can handle 2-Ohms...if they are willing to give a 20-20KHz spec on 2-Ohms it can probably handle it just fine...if they only like the 1KHz spec, it won't necessarily catch fire but you are bound to be heating it up a bit.

As to surround speakers, you should ALWAYS avoid series connections, IF your amplifier can handle the load. A series connection will introduce a sloppy bass response since this series bass will be very signifigant to the damping factor (a normally worthless figure in amplifiers). An experiment to try at home is to take an 8-Ohm resistor and put it in series with your speakers...listen for the difference in the bass response (some people actually prefer the sound).

Steve

------------------
"Old projectionists never die, they just changeover!"

Posted by Randy Stankey (Member # 64) on 05-04-2001, 10:18 PM:

I stand corrected, then. I know that even though some amplifiers say they'll run at a certain, low impedance they can't take it for long. I guess the answer is to check your amplifier specs.

Still, I stand by what I said earlier. You should at least check to be sure that you don't have an impedance problem.

Darryl;

Yes, I was in Salisbury just yesterday. I talked to Don. He had nothing but good things to say about you... no kidding! I was telling him that there are certain theatres that I NEVER hear from and that when I pick up my phone and see the name of one of these theatres displayed I say to myself, "Oh, Sh**! What's wrong?!" Don said that when Bob D. comes to your theatre it's almost like a vacation for him because there's nothing for him to do!

Posted by Mark Gulbrandsen (Member # 72) on 05-05-2001, 10:57 PM:

Randy said "Still, I stand by what I said earlier. You should at least check to be sure that you don't have an impedance problem."

I totally agree with him on this, especially if you are having failures on the surround channels. I have found many an improperly set up surround channel sometimes that went under 1 ohm! These channels even if configured properly can be overly taxing on an amplifier and hardly ever have a constant impedance curve. You may want to be sure that its wired correctly and that there are no blown tweeters. Blown tweeters that have shorted can cause alot of amplifiers to become unstable and eventually fail. I used to sell alot of Crown amps and also had a number fail and I stopped selling them. They do make a few neat, handy modules though(PIP-RPA for instance) that are useful for PA purposes and or background music in theater lobbies and I continue to use them for that. Have not had a failure there. They are all 70 volt systems though.
Mark @ GTS

Posted by Gordon McLeod (Member # 33) on 05-06-2001, 10:37 AM:

One other thing is make sure the vents on the sides of the amps are unubstructed by cabling so the air can get out. Also is the rack itself allowing for air to escape. Remeber the area of the inlets must have at least that much outlet space or airflow restricitons will occur.
I much prefer QSC's method blow the air out into the room from the amp

Posted by Steve Guttag (Member # 268) on 05-06-2001, 11:32 AM:

Many Crown amplifiers can actually do 1-Ohm in their goofy parallel mode (both outputs are strapped together)...not that I think this is the best situation but they are rated for it (at least the Macro-Techs are)....

I agree with the others on checking the impedance (or even just the resistance can tell you pretty much what is going on)...a proper impedance bridge can let you see what is going on by frequency though.

Another thing to remember when running surrounds at these low impedances is to have large enouch wire to handle the current load and not contribute significantly to the overall resistance of the system. If your typical surround speaker can take 100 watts (at 8-Ohms) then there will be at maximum just over 3.5 amps on the wire...Your wire needed be very large to handle the current load though you don't want it so small it acts too much like a resistor. However, if you take advantage of these new amps and run say 400 watts at 2-Ohms (4 100-Watt speakers in parallel) then the current leaving the amplifier jumps to over 14 amps! This puts you at 14awg for current considerations. If you use the QSC DCA-1222 with 5 8-Ohm speakers you now are up to nearly 18 Amps...and 12awg minimum.

The above should point out why one should use "home-run" lines rather than a series/parallel in the theatre. Power though the wire is expressed as current^2 x Resistance of the wire. Since current causes the power loss in the wire itself to move with the square of the current, it is best to have the current in the wire as low as possible since it is the dominent factor.

Steve

------------------
"Old projectionists never die, they just changeover!"

Posted by Joe Schmidt (Member # 822) on 05-06-2001, 11:28 PM:

Well, if you need 2 ohms you could change to McIntosh power amps. Some models with the autoformer have a 2-ohm tap and can give full rated power at any of the taps.

And the resistor thing is just what I am always bitching about. The bean counter wants 1/2 watt to save pennies when a good design will use 1-watt resistors minimum. Like in the Simplex XL tube systems.

Don't these people know? at the manufacturing level the cost difference between 1/2 & 1-watt resistors is incidental. You buy 'em in packs of 1000 !!!

Posted by James R. Hammonds, Jr (Member # 673) on 05-07-2001, 02:39 AM:

You know, it's funny that you guys keep bringing up QSC. We have those at one of our newer theatres and I had to change two of them in the year that I was there VS. who knows how many at my current location.

I told my manager that we should find some way to convince corporate office to change them all out, but we both know that's about as likely as them giving up on SDDS.

Posted by Gordon McLeod (Member # 33) on 05-07-2001, 09:25 PM:

2 amps you had to change out of how many and why did they fail.
Last year I had to change 1 QSC out several hundred we installed. Both failed due to a lightning strike.
Lets see I can't keep track of how many crowns I had to replace

Posted by John Gordon (Member # 523) on 05-08-2001, 11:40 AM:

As for the spacing question, at least with QSC amplifiers, you do not need to space them. If you have plenty of space in the rack and you want a symmetrical look, then go ahead and leave a space. It's your choice, but there is nothing wrong with stacking QSC amps. As mentioned above, all QSC amps air flow is from rear to front.

As others have said, it is a good idea to check the wiring to see what your load is. Just as SteveG noted above, we also recommend a parrallel wiring scheme for surrounds, for reasons already stated.

James, why did you have to change those two amplifiers. Which two were they? Were they new or old? If they were older models, did you have a regular preventative maintence schedule? Blowing out the dust that will collect in amps with an air hose, even once a year, on a regular basis is always a good idea, and highly recommended, as well as routine cleaning?

Another thing to check is the mode of the amp itself. Is the amp in bridge mode when it should be in stereo? Is it parallel when it should be in bridge, etc.? One little switch in the wrong position can cause the potential for major problems, not to meniton damage.

------------------
John Gordon
Cinema Applications Engineer
QSC Audio Products, Inc.
1675 MacArthur Blvd.
Costa Mesa, CA 92626
(714) 957-7100
john_gordon@qscaudio.com
http://www.qscaudio.com

Posted by James R. Hammonds, Jr (Member # 673) on 05-08-2001, 12:32 PM:

I dont remember exactly how many amps we had of the QSCs in our theatre, I'd guess it was in the neighborhood of about 125, since we had 24 auditoriums and we had 6 channels per house, with a mix between DCA series with one or two USA series.
It was the DCA series that I had to change out.
As far as the reson they, went out, I am not sure.
All I know is that we would completely lose whatever channel that amplifier supplied and that amplifier was sent back.
Also, since the theatre was new, the ams were installed new.
At one point, QSC had come and changed out all of the amps because some theatres were having problems with some of theirs.

The point I was trying to make was that the QSC amps were far more dependable (in my opinion) than the CROWNS.

Posted by John Gordon (Member # 523) on 05-10-2001, 12:25 PM:

Hello James,
I was the QSC person who came to swap out your DCA amps. As to why, one of our venders supplied us a bad part. It was not discoverd until after amps were shipped and in the field. The problem did not occur during testing--of which every QSC amp is tested before it is shipped--but would show up later, and as you stated, the channel would just die.
We took the approach of changing all your amps, not because they were all bad, but because they were suspect of having a bad part.

------------------
John Gordon
Cinema Applications Engineer
QSC Audio Products, Inc.
1675 MacArthur Blvd.
Costa Mesa, CA 92626
(714) 957-7100
john_gordon@qscaudio.com
http://www.qscaudio.com

Posted by John Pytlak (Member # 331) on 05-10-2001, 12:37 PM:

John Gordon of QSC said: "We took the approach of changing all your amps, not because they were all bad, but because they were suspect of having a bad part."

"Amplifiers Done RIGHT"!

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

Posted by Brad Miller (Member # 2) on 05-10-2001, 03:10 PM:

Now THAT is customer service!!!

Posted by Gordon McLeod (Member # 33) on 05-10-2001, 03:57 PM:

It would be nice if other manufactures stood behind there products to that extent with that sort of service

Posted by James R. Hammonds, Jr (Member # 673) on 05-11-2001, 02:36 AM:

Yeah, I remember you, John Gordon.
And come to think of it, I think I do remember mention of wthe bad part you were talking about.
I wasnt too concerned with it that day because I was too busy biulding up prints.
All I remember was that there was a problem, you guys were fixing it, and all 24 houses had to be done that night.
John Pytlak, Brad Miller, and Gordon McCleod are all right.
When I was told about it, I remember thinking "Well thats cool of them" which really is an understatement.
Not only did he change out all the amps, but he gave all of the projectionists that were on duty QSC keychains and took the guys who stayed overnight to help out for ice cream.
Hell, he even offered to buy me some even though I wasnt staying overnight, but I declined because I wasnt in an ice creamy mood.

Posted by Rory Burke (Member # 512) on 05-11-2001, 04:01 AM:

Should I be so lucky to have all my amps changed just because.....I said am I dreaming????? WOW NICE JOB!!

Amp spacing...well how bout this....Typical Mexican booths are NOT air conditioned and are heated damp "RECTIFIER BLOWING" environments. Amp spacing is definately recomended as to ensure reliability beyond the manufaturer suggestions but is not really a scenario common to most of us. Think of summer time during the night when the concrete or pavement still radiates heat x 2 = Mexican booth. Makes for tough B-chains cause the next morning everything changes. My hats always off to those projectionists. ohhh and the film condition...well you can imagine...ouch!

Rory